Audio-data are being transmitted in a compressed way conforming to psychoacoustic principles and decoded by the receiver. Re-compressing of these decoded data conforming to psychoacoustic principles will lead to loss of quality. This loss can be omitted, if the parameters of the original compression can be detected and re-used, see e.g. EP-A-0746913.
A method for the detection of these parameters for codecs which use spectra, is testwise calculation of spectra with different parameters. If the parameters of analysis (start of transformation, window type, length of spectrum) match the codec parameters, the spectrum has a quantized appearance. Because of calculation impreciseness or different calculation precision of the codec and of the analysis calculation, the quantized spectrum will look slightly noisy, i.e. the amplitudes of the same level of quantization will NOT have the same magnitude.
The difficulty is to distinguish, whether a spectrum has a “quantized character”, so that different parameters of the spectrum can be analyzed concerning their impact on the quantization of the spectrum.
Because compression goes along with a vanishing of certain lines of the spectrum (cf. in Herre, Jürgen; Michael Schug: Analysis of Decompressed Audio—The “Inverse Decoder”, 109th AES Convention, Los Angeles 2000) those spectral lines are being counted, which are smaller than a threshold value.